Current transient document technology enables images written on an erasable medium based on photochromic color change of photochromic materials, such as spiropyrans.
Spiropyrans (SPs) undergo reversible color change when irradiated at an appropriate wavelength. Typically, an ultraviolet (UV) light exposure converts SPs from a colorless state to a colored state of merocyanine. The colored state of merocyanine reverts back to the colorless state of spiropyran upon application of a thermal treatment as described by the following:
This color change is caused by a chemical structural change that involves absorption spectral change in the visible region. The color conversion and the color reversion are used to write and/or erase images.
Images are written visibly by a color contrast between an image and the surrounding background regions and are erased by removal of such color contrast. Visible images on conventional erasable media are readable for a few hours such as about 2-4 hours under ambient light conditions, and are often self-erased with no effort from the user and ready to be written or used again with new images in 24 hours.
On the other hand, longer image lifetimes and erase-on-demand capabilities are also desirable for erasable media. For example, it would be desirable for information on an erasable medium to be readable for as long as required by users, then erased or re-imaged on demand.
Thus, there is a need to overcome these and other problems of the prior art and to provide compositions and methods of erasable media having longer image lifetime and longer image readability.